A permanent magnet machine includes a rotor having a core that supports mounted permanent magnets. The rotor is surrounded by a stator that includes static coils. Interaction of a winding magneto motive force, (MMF) produced by the static coils, with the permanent magnets generates a rotary driving force. Based on how the permanent magnets are mounted, permanent magnet motors are classified as either surface-mounted permanent magnet motors or embedded permanent magnet motors. The embedded permanent magnet motor includes a plurality of permanent magnets mounted in the rotor core, while the surface-mounted permanent magnet motor includes permanent magnets mounted on the rotor core surface. The embedded permanent magnet motor is consequently used when high speed rotation is needed. Furthermore, the embedded permanent magnet machine has favorable characteristics such as good high speed power and efficiency and low spin loss, which makes the machine favorable for many applications including hybrid and fuel cell electric vehicle applications.
FIG. 1 is a cross-sectional view depicting a conventional embedded permanent magnet motor 10. The motor 10 is further classified as a concentrated winding motor since it includes concentrated pairs of coils in a stator that is disposed about the rotor. This is in contrast to a coil distribution motor in which coils are evenly distributed in a stator. The motor 10 includes a stator 11 on which coils 20 are wound, and a rotor 14 rotatably disposed within the stator 11.
The stator 11 includes a stator body 12 that is formed by stacking a plurality of magnetic steel sheets that, when stacked, together form the shape of a cylinder having a hollow core. A plurality of slots 13 are formed in the stator body 12 and are arranged in a circumferential arrangement to support the coils 20 therein. The stator body also includes a plurality of teeth 19 that are disposed alongside the slots 13.
The rotor 14 includes a rotor core 15 that is formed by stacking a plurality of magnetic steel sheets that, when stacked, together form the shape of a cylinder. The rotor core 15 is disposed in the stator body hollow core, while being spaced a predetermined distance from the stator body 12 so that a gap 21 is formed between the stator body 12 and the rotor core 15. A plurality of insertion holes 16 are formed in the rotor core 15 and are arranged in a circumferential arrangement to support a plurality of permanent magnets 17 that are inserted therein. A rotary shaft 18 is inserted in a hollow region formed at the center of the rotor 14, which rotates together with the rotary shaft 18.
When electric current is supplied to the coils 20 wound on the slots 13 of the stator 11, polarities of the coils are sequentially changed. A rotary magnetic field is consequently generated at the stator teeth 19 and a magnetic field is further generated at the rotor 14 in which the permanent magnets 17 are embedded. The magnetic field of the rotor 14 follows the rotary magnetic field generated at the stator teeth 19. As a result rotation of the rotor 14 generates a rotary driving force.
Elevated operating temperature environments for permanent magnet motors in automobiles, and particularly for hybrid applications, along with a high demagnification field that exists during machine operation, makes the permanent magnet motors vulnerable to demagnetization. To prevent demagnetization, high temperature magnets are frequently selected in permanent magnet motors when the machine is operated at high temperatures. However, for high energy rare earth magnets such as neodymium iron boron or samarium cobalt magnets, the magnet strength reduces as the operational temperature increases. Consequently, designers often choose a lower strength magnet that can withstand the demagnetization field that exists at elevated temperatures.
Accordingly, it is desirable to provide a permanent magnet that has high strength properties. In addition, it is desirable to provide a permanent magnet that is capable of withstanding the demagnetization field existing in the operating environment of a permanent magnet motor. Furthermore, other desirable features and characteristics of the present invention will become apparent from the subsequent detailed description and the appended claims, taken in conjunction with the accompanying drawings and the foregoing technical field and background.